This invention relates generally to a target support apparatus. More particularly, it relates to an assembly for supporting and varying the attitude of a target (or target model), such as an aircraft or a missile, while performing radar cross section measurements thereon.
The ability to make accurate and therefore meaningful and useful measurements of the signals reflected from targets which are illuminated by radar signals is dependent upon the capability of reducing spurious reflections introduced into the measured return signal from objects in the immediate vicinity of the target, such as the target support device.
The accuracy of such measurements is known to be directly related to the radar cross section (sometimes abbreviated herein as RCS) of the target in comparison to the RCS of the target support device and other background objects.
The relationship of the contributions of the target signal and background signals in a measured radar return signal can be expressed by the following equation: EQU .sigma..sub.m /.sigma..sub.t).sub..+-. =1+(.sigma..sub..beta. /.sigma..sub.t).+-.2(.sigma..sub..beta. /.sigma..sub.t).sup.1/2
where:
.sigma..sub.m =measured RCS PA1 .sigma..sub.t =target RCS PA1 .sigma..sub..beta. =background RCS
and .+-. refers to the upper and lower bounds of the measurement error respectively.
For example, for equal background and target signal powers, (proportional to their RCS's) the value measured for .sigma..sub.m can be as much as four times greater than the actual target value and the minimum value observed can in fact be zero. The actual measurement error (between the bounds) is dependent upon the relative phases of the target and the background signals. The principal contributor to the background signal can be, and quite often is, the target support structure.
Additional background information concerning RCS measurements of targets can be found in various publications, including the following: "The Radar Handbook", Merrill Skolnik, McGraw Hill, 1970; "Scattering Experiments at the Ipswich Electromagnetics Facility: Calibration with Perfectly Conducting Spheres", Robert V. McGahan, RADC-TR-83-181, August 1983; and "Introduction to Radar Cross Section Measurements", P. Blacksmith, Jr. et al, Proc. IEEE, vol. 53, August 1965.
In monostatic type radar measurements of low radar cross section targets, where by definition the radar transmits and receives electromagnetic signals at substantially the same angular position with respect to the target, the target support structure of choice is usually a single columnar support having an ogival cross section. The ogival column is tilted forward at some angle from the vertical to reduce residual scattering from the support up to the target. The mechanism that rotates the target to various positions is mounted inside the ogive. A small diameter shaft extends from the drive motor through the top of the ogive and up to the target. This shape and geometry make the unwanted contribution of the background signal small enough to permit high accuracy RCS measurements. See for example, "A GTD Analysis of an Ogive Pedestal"; Kim-Yue Albert Lai and N. D. Burnside, T. R. 716748-8, Ohio State University; and "Antenna and Radar Cross Section Positioning Systems", Orbit Advanced Systems 1987 Catalogue, pp 55-57.
In bistatic radar measurements of targets however, the radar transmitter and receiver are located at different angular positions with respect to the target, and the ogival shape of the aforementioned support column produces a very large cross section at some bistatic angles that results in a very large background signal. This large background signal exerts such an overriding influence on the measured signal that the measurement accuracy is unacceptably poor. This is particularly true when measuring the responses of small bistatic RCS target or targets with deep nulls in their bistatic RCS patterns.
For many bistatic RCS measurements, the target support structure of choice is presently a single column of circular cross section, i.e., a long thin cylinder, since it provides more uniform scattering of the radar signal with various bistatic angles, than does the ogive. The target positioning mechanism is mounted inside the cylinder and connected to the target by a shaft, just as with the ogive support. In many applications, however, the size of the cylindrical support column is such that the bistatic scattering from the cylindrical column is too high, varies with bistatic angle, and may even exceed the monostatic value for the cylinder. This severely limits the minimum bistatic RCS of targets that can be measured accurately with such a structure even when the cylinder is coated with radar absorbing material to limit its reflectivity.
In order to decrease the bistatic reflection, the single cylindrical support column can be tipped at an angle from the vertical and rotated about the vertical axis to take advantage of the cylinder's highly lobed elevation plane scattering pattern. The energy scattered by the tipped support in the direction of the receiving antenna (bistatic angle) is then monitored as the support is rotated until a point is found where the signal is minimized. This point corresponds to a null in the scattering pattern of the support. This adjustment insures that the target to be measured, (which target is mounted on the shaft extending from the support cylinder only after this adjustment is made), is little affected by the energy scattered from the support structure directly toward the receiving antenna.
This cylindrical target support structure and technique however, has the associated disadvantage of relying on the aforementioned elevation plane nulls. Because the support column is much longer than the wavelength of the radar signal, the angular separations of the nulls are very small and their depths are very sensitive functions of the rotation and tip angles of the cylindrical support. Thus, this structure requires very rigid and stable components to prevent wind and temperature variations from moving the position of the null and degrading its depth during a measurement period. Also, the bandwidth of the nulls is narrow because of the long support cylinder, and moderate changes in the measurement frequency will require reestablishing the null by again rotating the tipped cylinder. The characteristics of the nulls are fixed by the structure and cannot be changed.
There are other subtle disadvantages to tipping the support column. One is that for certain angles, there can be a significant scattering component from the tipped column up to the target mounted above it. This spurious reflection corrupts the incident target signal and impairs the measured results even if the energy reflected from the support directly to the receiving antenna is zero. Another subtle disadvantage is that reflections down from the tipped cylinder can reflect from the ground and back up to the target also corrupting the incident field. Yet another is that tipping the cylinder decreases the height of the target above the ground, thereby increasing the possibility of target-ground interactions that could also degrade the measurements.